The present invention relates to a polarizing filter used for controlling a state of polarization of light to be dealt with, and an optical device using the polarizing filter in an optical technology field such as optical communication, an optical sensor, an optical measuring device, and so on.
In an optical system applied to optical fiber communication or the like, incident light propagated through an optical fiber often contains an s-polarized light component and a p-polarized light component in an unequal intensity ratio. It is therefore necessary to provide a polarizing/optical device having a function for compensating for such incident light into a state of non-polarized light equal in the intensity ratio of the s-polarized light component to the p-polarized light component or for polarizing non-polarized incident light to obtain a desired intensity ratio of the s-polarized light component to the p-polarized light component.
As the optical device for controlling a state of polarization of incident light, there is well known a polarization beam splitter having a configuration in which tens of layers of high-refractive-index materials and low-refractive-index materials are laminated alternately on a transparent prism or on a flat transparent substrate. The polarization beam splitter can transmit only the p-polarized light component while it can cut off the s-polarized light component perfectly because an optical multilayer film satisfying the Brewster condition is provided in the polarization beam splitter.
Because such a polarization beam splitter uses light reflected or transmitted at the Brewster angle, the polarization beam splitter is allowed to use light narrow in the wavelength range, but can obtain the excellent degree of polarization in each of reflected light and transmitted light, high mass-production characteristic and is inexpensive in cost, compared with a polarizer using birefringence of a crystal such as a Nicol prism. For this reason, the polarization beam splitter is used popularly.
Because the polarization beam splitter is a device provided for separating incident light into the s-polarized light component and the p-polarized light component perfectly, the polarization beam splitter is, however, unsuitable for compensation for polarized light to obtain light with a desired intensity ratio of the s-polarized light component to the p-polarized light component.
On the other hand, it is known that the transmittance ratio of the s-polarized light component to the p-polarized light component varies when light is made incident obliquely on a transparent flat plate such as a glass plate. A polarizing filter using this characteristic is more suitable for light compensation than the polarization beam splitter.
In order to make the degree of polarization high by using such a transparent flat plate, it was, however, necessary to make the incident angle (an angle between incident light and a line normal to the transparent flat plate on which the light is incident) large. When the transparent flat plate was used as an optical device, there was a problem that the size of a housing for the device became large. Moreover, when the incident angle was made large, it was necessary to use each of the s-polarized light component and the p-polarized light component in a low transmittance region. Inevitably, this caused a problem that the intensity of light taken out was reduced.